The invention relates to alkaline pulping, and particularly to the process stages following an alkaline cooking stage and prior to further delignification stages.
Alkaline pulping processes and especially kraft pulping are dominant in the production of cellulose, because alkaline pulping provides pulp fibers which are stronger than those from any other commercial pulping process. However, in industrial alkaline pulping processes fiber damage occurs. It has been found that laboratory pulp produced from the same chip lot normally shows superior strength compared to the commercial alkaline pulp. The present invention relates to an improved method for treating delignified lignocellulosic material after delignification in alkaline cooking liquor and cooling of the cooked material. The invention relates to a method whereby improved strength properties of cooked material are achieved compared to material that has been treated under normal industrial conditions after the end of alkaline cooking and cooling of the cooked material.
In the alkaline cooking processes, the lignocellulosic material reacts with alkaline cooking liquors for a certain time at a specified temperature. The cooking liquor can be kraft liquor, soda, alkaline sulfite, polysulfide, alkaline solvent or other modifications, e.g. including added anthraquinone. At the end of a cook, a specified degree of delignification being achieved, the cooking material is at high temperature and pressure inside a digester. This is true in both continuous and batch cooking processes.
The cooked material can then be cooled using cooler spent liquors to replace the hot spent liquor surrounding the delignified material inside the digester. This routinely occurs in the counter-current washing zone in many continuous digesters, but is less common in conventional batch digesters. With or without precooling, the delignified cellulosic material can be removed from the digester under pressure using a pipe to a receiving tank essentially at atmospheric pressure. Because of this, the cooked material experiences a large pressure and/or temperature drop in an highly alkaline environment via a series of transport and depressurising devices during its transfer from the digester to the receiving vessel. The outcome of this mechanical action during blow is usually inferior pulp and fiber strength compared to the strength potential of the lignocellulosic material. This was found, for example, by retrieving samples from baskets placed inside industrial conventional batch digesters. Reference pulp was thus obtained which had not experienced the vigorous treatment involved with blowing. This pulp showed a strength comparable to that of pulp from pilot digesters. Thus, it was concluded that the strength deficit occurred in the digester blow.
In the 1980""s, liquor displacement procedures in batch digesters were developed. This technology was driven by energy considerations, and also provided improved strength delivery of the delignified cellulosic material over cooking and the possibility to extend delignification by cooking. Thus, less fiber damage was induced in cooking and digester discharge. This was achieved by (1) modified cooking chemistry, (2) a uniform chemical and temperature profile in the digester, and (3) gentle discharge of the digester. Examples of gentle discharge techniques used in liquor displacement batch cooking are xe2x80x9ccold blowxe2x80x9d and pump discharge. In U.S. Pat. No. 4,814,042, a method to gently remove delignified cellulosic material from a digester at the end of an alkaline cook is described. The cooked material is cooled to below 100xc2x0 C., and the overpressure in the digester is essentially released to or near atmospheric pressure. The cellulosic material is then transferred as a fluid suspension to a receiving tank using a pump. Pumping is carried out at a controlled flow rate to reduce physical fiber damage compared to conventional discharge of digesters, resulting in improved strength properties in the pulp. The pump discharge technique is today routinely used in liquor displacement batch digesters at temperatures below 100xc2x0 C. to avoid large pressure differences between digester and receiving vessel, boiling of liquors in the pipe from the digester to the receiving vessel, boiling in the receiving vessel, cavitation in pumps etc. The reason for using temperatures below 100xc2x0 C. or close to 100xc2x0 C. is that it makes it possible to use atmospheric receiving tanks, which are less expensive than pressure vessels. This technique allows a low pressure difference between the digester and the receiving tank, which is advantageous for pulp strength. Another reason is that boiling at atmospheric pressure is avoided if temperatures below the boiling point of the liquor (below 100xc2x0 C.) are used. This lowers the amount of released odour gases. For these reasons, the discharge temperatures used for modern industrial liquor displacement batch digester fiberlines are typically 90 to 100xc2x0 C. Lower temperatures has so-far not been seen required. In continuous digesters, the discharge temperature is normally between 80 and 100xc2x0 C. when the digester is followed by a receiving tank or atmospheric diffusion washer. The temperature can be 80 to 120xc2x0 C. if the digester is followed by a receiving tank or a pressure diffusion washer. The optimal discharge temperature in continuous digesting followed by a diffusion washer is a consequence of the vigorous discharge from the continuous digester as the cooked material experience a large immediate pressure drop which creates e.g. foam, flashing and thereby determines the maximal operating temperature of the first washing equipment after the digester. The vigorous discharge and treatment in continuous digesting will also damage the fiber. Thus, high quality pulp is not produced in continuous cooking.
Another route of development has been to increase the temperature in washing of the cooked defiberised material. In washing, increased temperature improves drainability of water through the pulp mat and enhances leaching of dissolved material to the surrounding liquor. Conventional vacuum washers, being the most widely used equipment for brown-stock washing and in bleach plants, are being replaced by such new, more efficient washing equipment as pressure washers, drum displacement (DD) washers, wash presses and diffusion washers. Vacuum washers cannot normally be operated at temperatures above 85xc2x0 C. However, the new washing equipment mentioned above can be operated at temperatures above 85xc2x0 C. Thus, the latest development in washing technology enables use of higher temperatures and improved washing efficiency. The equipment involved essentially operates at atmospheric or higher pressure. Thus, vacuum is not used to generate a pressure difference over the pulp mat. The new washing technology also shows more efficient washing at the same temperature.
We have found that the pulp sampled from the inlet to the receiving tank when pump digester discharge is used at temperatures below 100xc2x0 C., which typically means between 100 and 90xc2x0 C., using an essentially depressurised digester, shows a strength delivery which is typically close to 100%. However, we have also found that this strength is typically not retained after further brownstock treatment, although the method described in above cited U.S. Pat. No. 4,814,042 and Tappi J., October 1987, p. 157-163 is used.
Thus, although a kraft digester is discharged gently, at a low flow velocity, the conditions in the process stages following discharge should also be taken into account to produce maximum pulp strength.
In normal practice according to the prior art, the cooked cellulosic material experiences a series of mechanical processing stages in depressurising devices, valves, agitators, separation devices and pumps before bleaching and delignifying with, e.g. oxygen, chlorine or chlorine dioxide-containing chemicals. We have found that significant strength losses can occur during hot treatment stages of the delignified cellulosic material following digester discharge, i.e. storage in the receiving tank at 90 to 100xc2x0 C., knotting, screening and washing.
After batch cooking and discharge, the pulp slurry is usually stored in a receiving tank at the resulting temperatures which is typically 90 to 100xc2x0 C., and subsequent treatment of the delignified cellulosic material occurs essentially at a pH above 10. This is evident by measurement of pH in the liquor surrounding the pulp at various points between alkaline cooling and the first delignification/bleaching stage. In normal washing after alkaline cooking, the pH remains alkaline (Pulp and Paper Chemistry and Chemical Technology, third edition, Volume 1, James P. Casey, ed., p. 446). The temperature also remain close to the boiling point of the liquor in digester discharging. In Tappi J. October 1989, p. 157-161 it is shown that the digester discharge temperature is in the range 85-100xc2x0 C. after a complete displacement of a batch digester with wash liquor. Typically, most of the material inside the digester is cooled to approximately 100xc2x0 C. after liquor displacement with wash liquor (p. 159). The temperature can also remain close to the boiling temperature in the intermediate stages if the counter-current liquors used in washing and dilution of the cooked material are not temperature-adjusted. For example, a system applying liquor-displacement cooking, discharge of the digester at 95xc2x0 C. to a storage tank, brownstock washing with pressure filters, screening and oxygen delignification at 100xc2x0 C. will show a temperature of approximately 95xc2x0 C. in the area between the digester and the first delignification/bleaching stage. However, we have observed that treatment of the cooked material at temperatures close to 100xc2x0 C. in single or several stages induced fiber damage which is a consequence of the combination of mechanical treatment and high temperature, alkalinity, ionic strength and impurities in the liquor within and surrounding the alkaline delignified cellulosic material after alkaline cooking. In an article by Cyr et al (Tappi J. October 1989 p. 162), it is stated that variables such as temperature, pressure drop, geometry of the blow system and the alkalinity of the spent liquor may alter the amount of fiber damage from mill to mill. The authors, however, suggest that appropriate pulp suspension velocity in two-phase flow probably can minimize pulp strength losses in all cases.
However, as those skilled in art are aware, the alkaline pulp (brownstock) is processed further and stored after digester discharge, awaiting further delignification and/or bleaching of the impure alkaline pulp. Thus, there are numerous process stages involving mechanical treatment that can still damage the fiber. The process conditions as the combination of temperature, pH and ionic strength are also important for the pulp strength outcome.
Brownstock washing is a dominant operation between the cooking and further delignification stages. Many factors influence the operation of washers. The choice of operating temperature is important, since higher temperature reduces the liquor viscosity and thus improves liquor drainage. As stated above, the most widely used equipment is the conventional vacuum washer. In a vacuum washer, however, too high a temperature increases the vapor pressure and thus reduces the vacuum in the drop leg of a vacuum washer. In a conventional brownstock washing system with rotary vacuum washers, the temperature is maintained by using hot water at about 60-70xc2x0 C. in the final stage washer (Pulp and Paper Chemistry and Chemical Technology, third edition, Volume 1, James P. Casey, ed., p. 448). Conventional vacuum washers are therefore being replaced by pressure washers, drum displacement (DD) washers, wash presses, atmospheric diffusion washers and pressure diffusion washers, as modern washing equipment can be operated at higher temperatures and are more efficient washers.
The reasons for maintaining high temperatures in intermediate stages according to the prior art are usually found in improved washing efficiency and capacity when e.g. a pressure filter is operated at higher temperatures. If higher temperatures are allowed, no pre-cooling of pulp or liquors is required. Another reason for using high temperatures is the increased use of oxygen delignification. Oxygen delignification is typically carried out at temperatures between 90 to 105xc2x0 C., and a consequence of this is an increase in the temperature of the brownstock washing (i.e. pre-oxygen delignification washing) stage, as the filtrates produced in post-oxygen delignification washing are used in brownstock washing.
Pulp consistency typically varies between 1.5 and 35% between digester discharge and the first subsequent delignification stage.
The primary objective of the present invention is to provide a method for treating delignified cellulosic material during the process stages between the end of an alkaline cooking stage and the beginning of a stage for further delignification, thereby preventing the pulp fibers from suffering severe damage. A method according to the present invention furnishes pulp that can be used for the manufacture of paper materials which have superior strength properties relative to materials made from pulp produced according to the prior art.